2D children not rendered when set visible

[typically]

When a Shape3DInt is set visible, it is not rendered in a 2D window (requires a manual "regen"). This is because no 2D child shape is present to notify the 2D window.

Possible fixes:

• Add ShapeShown event, fire this on the ShapeModel, and subsequently on all windows in the InterfaceDisplayPanel.
• Retain 2D windows as listeners on the Shape3DInt object
• Do not remove a 2D child when Shape3DInt is set invisible (retain child and fire an event when IsVisible is set to true)